1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor film having a crystal structure and formed on a substrate having an insulating surface, and a method of fabricating a semiconductor device using the semiconductor film as an active layer. Particularly, the present invention relates to a method of fabricating a thin film transistor in which an active layer is formed of a crystalline semiconductor layer. Incidentally, in the present specification, the term “semiconductor device” indicates all devices capable of functioning by using semiconductor characteristics, and includes, in its category, an electro-optical device typified by an active matrix type liquid crystal display device formed by using thin film transistors, and an electronic equipment incorporating that kind of electro-optical device as a part.
2. Description of the Related Art
There has been developed a thin film transistor (hereinafter referred to as a TFT) in which an amorphous semiconductor layer is formed on a translucent substrate having an insulating surface and a crystalline semiconductor layer crystallized by a laser annealing method, heat annealing method or the like is made an active layer. As the insulating substrate, a glass substrate of barium borosilicate glass or alumino borosilicate glass is often used. Although such a glass substrate is inferior to a quartz substrate in heat resistance, it has merits that its market price is inexpensive and a large area substrate can be easily manufactured.
The laser annealing method is known as a crystallizing technique in which it is possible to crystallize an amorphous semiconductor layer by giving high energy to only the amorphous semiconductor layer without raising the temperature of a glass substrate very much. Particularly an excimer laser capable of obtaining short wavelength light having a wavelength of 400 nm or less and large output is regarded as most suitable in this usage. The laser annealing method using the excimer laser is carried out in such a manner that a laser beam is processed by an optical system into a spot shape or linear shape on a surface to be irradiated, and the surface to be irradiated on the substrate is scanned by the processed laser beam (irradiation position of the laser beam is moved relatively to the surface to be irradiated). For example, in an excimer laser annealing method using a linear laser beam, it is also possible to make laser annealing of all the surfaces to be irradiated by scanning only in the direction normal to its longitudinal direction, and is superior in productivity, so that it has become the mainstream of a manufacturing technique of a liquid crystal display device using TFTs. The technique enables a monolithic type liquid crystal display device in which TFTs (pixel TFTs) for forming a pixel portion and TFTs of a driving circuit provided at the periphery of the pixel portion are formed on one glass substrate.
However, a crystalline semiconductor layer fabricated by the laser annealing method is formed of an aggregation of plural crystal grains, and the positions and sizes of the crystal grains are random. TFTs fabricated on the glass substrate are formed such that the crystalline semiconductor layer is separated into an island-like pattern for the purpose of element separation. In that case, it was impossible to specify the positions and sizes of the crystal grains and form them. In the interface (crystal grain boundary) of the crystal grain, there is a cause to lower current transport characteristics of carriers because of a recombination center or trapping center due to an amorphous structure, crystal defect or the like, or the influence of a potential level at the crystal grain boundary. However, it has been hardly possible to form a channel formation region, in which the property of a crystal greatly influences the characteristics of a TFT, by a single crystal grain so as to exclude the influence of the crystal grain boundary. Thus, a TFT including an active layer of a crystalline silicon film and having characteristics comparable to those of a MOS transistor has not been obtained till today.
In order to solve such problems, an attempt to grow a large crystal grain has been made. For example, in ┌“High-Mobility Poly-Si Thin-Film Transistors Fabricated by a Novel Excimer Laser Crystallization Method”, K. Shimizu, O. Sugiura, and M. Matsumura, IEEE Transactions on Electron Devices vol. 40. No. 1, pp 112-117, 1993┘, there is a report on a laser annealing method in which a film of three-layer structure of Si/SiO2/Si is formed on a substrate, and an excimer laser beam is irradiated from both sides of a film side and a substrate side. This report discloses that according to this method, the size of a crystal grain can be enlarged by irradiation of a laser beam at predetermined energy intensity.
The above-mentioned method of Ishihara et al. is characterized in that heat characteristics of an under material of an amorphous silicon film are locally changed and the flow of heat to the substrate is controlled, so that a temperature gradient is caused. However, for that purpose, the three-layer structure of high melting point metal layer/silicon oxide layer/semiconductor film is formed on the glass substrate. Although it is possible to form a top gate type TFT by using the semiconductor film as an active layer in view of structure, since a parasitic capacitance is generated by the silicon oxide film provided between the semiconductor film and the high melting point metal layer, power consumption is increased and it becomes difficult to realize high speed operation of the TFT.
On the other hand, when the high melting point metal layer is made a gate electrode, it is conceivable that the method can be effectively applied to a bottom gate type or reverse stagger type TFT. However, in the foregoing three-layer structure, even if the thickness of the semiconductor film is omitted, with respect to the thickness of the high melting point metal layer and the silicon oxide layer, since the thickness suitable for a crystallizing step is not necessarily coincident with the thickness suitable for the characteristics as a TFT element, it is impossible to simultaneously satisfy both the optimum design in the crystallizing step and the optimum design in the element structure.
Besides, when the opaque high melting point metal layer is formed on the entire surface of the glass substrate, it is impossible to fabricate a transmission type liquid crystal display device. Although the high melting point metal layer is useful in that its thermal conductivity is high, since a chromium (Cr) film or titanium (Ti) film used as the high melting point metal material layer has high internal stress, there is a high possibility that a problem as to adhesiveness to the glass substrate occurs. Further, the influence of the internal stress is also exerted on the semiconductor film formed as the upper layer, and there is a high possibility that the stress functions as force to impart distortion to the formed crystalline semiconductor film.
On the other hand, in order to control a threshold voltage (hereinafter referred to as Vth) as an important characteristic parameter in a TFT within a predetermined range, in addition to valence electron control of the channel formation region, it is necessary to reduce the charged defect density of a base film and a gate insulating film formed of an insulating film to be in close contact with the active layer, or to consider the balance of the internal stress. To such requests, a material containing silicon as its constituent element, such as a silicon oxide film or a silicon nitride oxide film, has been suitable. Thus, there is a fear that the balance is lost by providing the high melting point metal layer to cause the temperature gradient.